FIG. 1 is a schematic cross-sectional view through an aircraft wing box. The wing box has a pair of C-section spars 1, 2 and upper and lower covers 3, 4 which define the walls of a fuel tank containing fuel 5. FIG. 2 shows in detail the joint between a cap 6 of the spar 1 and the upper cover 3.
The spar cap 6 is formed from a series of plies of carbon fibre-reinforced epoxy resin. Each ply comprises a plurality of uni-directional carbon fibres impregnated with an epoxy resin matrix material. Some of the layers of carbon fibre 7 are shown in FIG. 2, and these fibres terminate at a cut edge 8. If a lightning strike 9 hits the upper cover 3, then current 10 tends to run along a bronze mesh (not shown) on the external surface of the cover 3, down the metal fasteners 12 which join the spar cap to the cover, and along the carbon fibre layers in the spar cap 6 up to the cut edge 8 which is a potential source of edge glow 13. Note that the epoxy resin between the fibres is highly resistive so current tends not to flow between adjacent plies of the composite material.
One solution to the problem of edge glow is presented in US2008/0128430. The cut laminate edge is sealed with an edge seal produced from a prepreg form using a thermosetting resin matched to the characteristics of the resin used in the laminate. The prepreg form can be applied to the cut laminate edge either before or after the laminate is cured, optionally with an epoxy film adhesive. The edge seal acts as a dielectric layer that both electrically insulates the cut laminate edge from the fuel and mechanically contains energetic particles produced at the edge due to lightning strikes or other sources of electrical charges.
A number of possible problems have been identified with the solution presented in US2008/0128430. Firstly the prepreg material which forms the edge seal is relatively brittle and, as such, there is a risk that it may shatter in the event of a lightning strike. Specifically, a lightning strike can result in the generation of high pressure gasses and/or plasma at the cut edge which may have the potential to shatter the relatively brittle edge seal. Secondly the bond between the prepreg material and the cut edge is relatively brittle, so the gasses and/or plasma may have the potential to cause the bond to shatter. Finally the U-shaped cross-sectional profile of the edge seal means that it is difficult to join the laminate to another component. Specifically: a) a joggle or cut-out must be formed in the laminate or the other component in order to accommodate one flange of the edge seal which is sandwiched between them; and b) the edge seal cannot be fitted after the laminate has been joined to the other component. Also, it is difficult to remove and replace the edge seal for inspection or repair purposes. This is because it is necessary to dismantle the joint in order to remove the seal, and because the nature of the adhesive bond between the cut edge and the prepreg makes it physically difficult to break.